There is a significant quality control problem associated with surface imperfections on magnetic disks. This typically occurs, for example, on nickel-plated aluminum substrates used in the manufacture of thin-film magnetic media, but may be a problem with respect to any area where a smooth surface is desired. Typical surface defects include pits, dirt, dust, oil, stains, fingerprints and the like. Defects on the surface of rigid magnetic media are often a result of an impingement onto the surface or a tearing of material away from the surface. These types of defects can be very large scratches or gouges on the surface or very small (5 um and smaller) tears or pricks on the surface. The large surface defects, because of their size and scattering properties, are readily distinguishable through sophisticated data processing performed on light reflected from the surface of the medium under test by an inspection apparatus which includes a light source directed at the disk. Small surface defects (5 um and smaller) have not been so readily detectable, and even when detected, have been difficult to identify and classify. For example, the systems described in U.S. Pat. Nos. 4,794,264 and 4,794,265, entitled "SURFACE DEFECT DETECTION AND CONFIRMATION SYSTEM AND METHOD" and "SURFACE PIT DETECTION SYSTEM AND METHOD", respectively, issued to Quackenbos et al. describe systems for detecting pits on a smooth surface by irradiating an area of the surface. Two sensors separately detect radiation scattered from the surface. One sensor detects radiation scattered in a near-specular region (40-100 milliradians or 2.29-5.73 degrees), while a second sensor detects radiation scattered in a far-specular region (greater than 100 milliradians or 5.73 degrees). The near-specular signal is normalized with respect to the far-specular signal to indicate a pit. The Quackenbos devices lack any ways to distinguish between a surface depression, i.e., a pit, and a surface protrusion, i.e., a bump. Quackenbos does not distinguish surface protrusions of a smooth nature (i.e., bumps) from more jagged contaminants. Quackenbos also makes the assumption that surface depressions or "pits" do not have far-specular reflection patterns, which has proved to be a limiting and problematic assumption. Additionally, the Quackenbos devices lack any means to determine the slope of the defect from which the light has scattered.
Thus, there is a need in the magnetic disk drive industry for a noncontact optical inspection instrument which is capable of detecting defects in the surfaces of polished magnetic disk substrates. This instrument must be sensitive, fast and inexpensive and must be capable of detecting surface defects and estimating the size of these defects. This instrument must also be able to distinguish between various kinds of defects such as bumps, pits and scratches and also between these defects and surface contaminants such as particles and stains. The instrument should also be able to determine the slope of the defect from which the light has scattered.